Current management system for a stimulation output stage of an implantable neurostimulation system

ABSTRACT

A current management system for use in the stimulation output stage of a neurostimulation system can be programmed to steer different amounts of current through different stimulation electrodes to vary how strongly the tissue adjacent each electrode is stimulated during a particular programmed stimulation episode. An stimulation electrode drive circuit associated with each electrode that is available for stimulation allows independent control of the flow of current through that electrode. A reference electrode is provided in the circuit to source or sink current as necessary to balance the currents going into and out of the patient, so that no stimulation electrode is required to serve that purpose. More specifically, by configuring the circuit to maintain a constant potential at the reference electrode (e.g., a potential that is approximately half way between a top and bottom voltage rail), the reference electrode will source or sink currents as necessary to cause the net current flow into the patient to be equal to the net current flowing out of the patient, thus satisfying Kirchhoff&#39;s current law.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/843,785, entitled “Current Management System for a Stimulation OutputStage of an Implantable Neurostimulation System” and filed on Sep. 9,2015, which is a divisional of U.S. application Ser. No. 12/886,279,entitled “Current Management System for a Stimulation Output Stage of anImplantable Neurostimulation System” and filed on Sep. 20, 2010, nowU.S. Pat. No. 9,155,891, each of which is expressly incorporated byreference herein in its entirety.

GOVERNMENT CONTRACT

The U.S. Government has a paid-up license in this invention and theright in limited circumstances to require the patent owner to licenseothers on reasonable terms as provided for by the terms of theDepartment of Commerce, National Institutes of Standards and Technology,Cooperative Agreement No. 70NANB7H7001.

FIELD

The disclosed embodiments relate to current control systems, and moreparticularly to current control systems in implantable neurostimulationsystems that can deliver electrical stimulation as a form of therapy toa patient through one or more electrodes.

BACKGROUND

Neurostimulation systems, and increasingly implantable neurostimulationsystems, are used to treat various neurological diseases and otherneurological disorders, such as epilepsy, movement disorders (e.g.,Parkinson's disease) and chronic pain. Research is ongoing concerninguse of implantable neurostimulation systems to treat psychologicaldisorders (e.g., depression), headaches and Alzheimer's disease and tofacilitate stroke recovery.

A typical neurostimulation system comprises a stimulation source, suchas a pulse generator, and a stimulation output (or therapy output) stagethrough which a form of stimulation (e.g., electric current or voltage)can be delivered to target neural tissue. The output stage is incommunication with a plurality of electrodes that are disposed in ornear the target brain tissue. For example, a brain lead can be used toconnect multiple electrodes located on a distal end of the lead throughconductors to a proximal end of the lead which then can be connected tothe neurostimulator. The electrode-bearing leads may be designed so thatthe electrodes are intended to be placed on a surface of the brain(cortical strip electrodes on a cortical strip lead) or within the brain(deep brain electrodes on a deep brain or depth lead).

The stimulation to be delivered to the patient is typicallyprogrammable. For example, a neurostimulator may be loaded with a set ofprogrammed instructions that cause it to initiate a stimulation episodeaccording to a particular schedule or in response to some predeterminedphysiological condition or conditions or a neurological event or events.Various parameters related to the stimulation episodes also may bepredetermined by programming, for example, whether the stimulationepisode consists of pulsatile or non-pulsatile stimulation, and, ifpulsatile, how many pulses in a burst, how many bursts within theepisode, and the amplitude, frequency or pulse-to-pulse intervals withina burst all may be programmable. In addition, the electrodes among theelectrodes available for stimulation through which a stimulation episodeis delivered can be preselected by programming. For instance, there maybe four electrodes on the distal end of a deep brain lead, all of whichare available for use in a stimulation episode, and the neurostimulationsystem may be programmable to deliver stimulation for a givenstimulation episode in a bipolar fashion from the most proximalelectrode on the lead to the next most proximal electrode on the lead,or between the first three most proximal electrodes and the most distalelectrode.

As will be appreciated by those with skill in the art, there may becircumstances in which it would be desirable to be able to program aneurostimulation system so that different amounts of current can bedelivered through different stimulation electrodes at a given instant,for example, to deliver stronger stimulation to the location adjacentone of the electrodes than the stimulation delivered at the location(s)adjacent the other(s). Thus, what is needed is a device and methodassociated with a neurostimulation system for independently controllingthe current that is delivered through each of a plurality of electrodesavailable for stimulation.

SUMMARY

A current management system for the stimulation output stage of aneurostimulation system is disclosed in which it is possible toindependently control the current flow through each of a plurality ofelectrodes in a set of stimulation electrodes so that, for example, thestrength of the stimulation near a given one of the electrodes in theset can be reliably estimated and the current delivered can be steeredamong several different stimulation electrodes.

In one variation, the neurostimulation system is implantable in a humanpatient and the current management system provides programmedinstructions from which digital control and timing signals are derivedthat determine what function which stimulation electrode will have atwhat time during a stimulation episode; the functions for eachstimulation electrode are implemented by a stimulation electrode drivecircuit for each electrode and including causing the stimulationelectrode to source current into the patient, sink current out of thepatient, present a high impedance (e.g., turn the electrode “off”), andprovide a short circuit to ground. The reservoir voltage for eachelectrode drive circuit allows it to cause its associated stimulationelectrode to source or sink current within limits (e.g., the limits of atop and bottom voltage rail). The total amount of current being sourcedor sunk through the stimulation electrodes at any given time in thestimulation electrode is balanced by an equal amount of current sunk orsourced through a reference electrode, by maintaining the referenceelectrode at a constant voltage.

In some variations, a bias circuit is provided, for example, for eachstimulation electrode drive circuit, to convert the programmedinstructions in the form of digital control and timing signals into adigital enable signal and an analog signal containing information toestablish the reference current for a given sourcing or sinkingfunction.

In some variations, the voltages in the current management system areabove and below ground, for example, a top voltage rail is above ground,a bottom voltage rail is below ground, and the reference electrode ismaintained at around ground potential. In some variations, all of thevoltages in the current management system are at or above ground, forexample, a bottom rail voltage is around ground, and the referenceelectrode drive circuit sets the reference electrode voltage at amidrail voltage between ground and a top rail voltage.

The voltages used by the current management system may be derived from apower source in a neurostimulator, such a primary cell battery or arechargeable battery. The voltages may be increased by boost convertercircuits (e.g., voltages of up to +16 V may be derived from a 3 Vbattery).

In other variations, a “TILT” signal is generated whenever a programmedamount of current cannot be sourced or sunk through a stimulationelectrode because the limits defined by the top and bottom voltage railsare exceeded.

In still other variations, the voltages in the current management systemare automatically adjustable within predetermined not-to-exceed valueswhenever a programmed amount of current cannot be sourced or sunkthrough a stimulation electrode because the limits defined by the topand bottom voltage rails are exceeded, the adjustment being such toallow the programmed amount of current to be delivered so long as theprogrammed amount of current can be delivered within the not-to-exceedvalues.

Also described herein is a method for steering the amount of current tobe sourced or sunk to a patient through each of a plurality ofstimulation electrodes under the control of a neurostimulator in aneurostimulation system. A stimulation electrode drive circuit isprovided for each stimulation electrode and is configurable to cause theassociated stimulation electrode to perform one of four functions at anygiven time period in a stimulation episode, namely, sourcing currentinto the patient, sinking current out of the patient, presenting a highimpedance, or providing a short circuit to ground. Programmedinstructions are provided in the form of digital control and timingsignals to each stimulation electrode drive circuit to cause theassociated stimulation electrode to perform one or more of the fourfunctions at selected time periods during the stimulation episode. Avoltage reservoir for the stimulation electrode drive circuits isprovided for when the stimulation electrode drive circuits areprogrammed to source or sink currents. A reference electrode drivecircuit is provided that maintains a housing of the neurostimulator at aconstant reference voltage during the stimulation episode, which causesthe reference electrode to source or sink an amount of current equal tothe total amount of current being sunk or sourced by the stimulationelectrodes at each selected time period. This allows the current sourcedor sunk through any one stimulation electrode to be independent of theamount of current being sourced or sunk through any other stimulationelectrode at any given time period in the stimulation episode.

The features and advantages described in the specification are not allinclusive and, in particular, many additional features and advantageswill be apparent to one of ordinary skill in the art in view of thedrawings, specification, and claims. Moreover, it should be noted thatthe language used in the specification has been principally selected forreadability and instructional purposes, and may not have been selectedto delineate or circumscribe the inventive subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating a current management systemfor a stimulation output stage of a neurostimulation system.

FIG. 2A is a graphical illustration of a control signal for astimulation electrode.

FIG. 2B is a graphical illustration of a timing signal for a stimulationelectrode.

FIG. 2C is a graphical illustration of the function of an electrodecorresponding to the control signal and timing signal of FIGS. 2A and2B.

FIG. 3 is schematic diagram of a prior art stimulation output stage fora neurostimulator.

FIG. 4 is a perspective view of a neurostimulation system implanted inthe head of a human patient.

FIG. 5 is a schematic diagram illustrating some of the circuit elementsfor a current management system for a stimulation output stage of aneurostimulation system where the potential at which the referenceelectrode is maintained is at or near ground potential.

FIG. 6A is a graphical illustration of a stimulation episode that mightbe used with a current management system for a stimulation output stageof a neurostimulation system.

FIG. 6B is a graphical illustration of the current passing through afirst stimulation electrode of a neurostimulation system during thestimulation episode of FIG. 6A.

FIG. 6C is a graphical illustration of the current passing through asecond stimulation electrode of a neurostimulation system during thestimulation episode of FIG. 6A.

FIG. 6D is a graphical illustration of the current passing through athird stimulation electrode of a neurostimulation system during thestimulation episode of FIG. 6A.

FIG. 6E is a graphical illustration of the current passing through areference electrode of a neurostimulation system during the stimulationepisode of FIG. 6A.

FIG. 7 is a graphical illustration the current passing through each offour stimulation electrodes and a reference electrode during twodifferent stimulation episodes.

FIG. 8 is a schematic diagram of a stimulation electrode drive circuitof a current management system of a neurostimulation system where thepotential at which the reference electrode is maintained is at or nearground potential.

FIG. 9 is a schematic diagram of illustrating some of the circuitelements for a current management system for a stimulation output stageof a neurostimulation system where the potential at which the referenceelectrode is maintained at a voltage that is between ground and apositive voltage.

DETAILED DESCRIPTION

Various embodiments are now described with reference to the figureswhere like reference numbers indicate identical or functionally similarelements. Also in the figures, the left most digits of each referencenumber corresponds to the figure in which the reference number is firstused.

Reference in the specification to “some embodiments” or “somevariations” means that a particular feature, structure, orcharacteristic described in connection with these embodiments orvariations is included in at least one embodiment or at least onevariation of the invention. The references of the phrase “in someembodiments” or “in some variations” in various places in thespecification are not necessarily all referring to the same embodimentor variation.

All publications, patents, and patent applications cited herein arehereby incorporated by reference in their entirety for all purposes tothe same extent as if each individual publication, patent, or patentapplication were specifically and individually indicated to be soincorporated by reference.

As used herein, the term “stimulation episode” refers to any instance inwhich electrical stimulation is delivered to a patient through one ormore electrodes configured for that purpose. A stimulation episode maybe characterized by one or more parameters that determine how long theepisode will last, of what sort of stimulation the episode will becomprised (e.g., pulsatile stimulation, non-pulsatile stimulation,pulsatile or nonpulsatile stimulation with a direct current component,etc.), how strong the stimulation delivered will be at any given pointin the episode (e.g., amplitude of the stimulation, duration of thestimulation); whether the stimulation will be delivered in bursts withina stimulation episode and, if so, the number, frequency and shape(morphology) of pulses within a burst, the delay between pulses within aburst or between bursts or between transitions from positive-goingpulses to negative-going pulses, etc. A stimulation episode may also bedefined with reference to which or how many electrodes in a set ofelectrodes will be used in delivering it. Although the term “stimulationepisode” is used herein almost exclusively with reference to electricalcurrent stimulation, the term should be understood to encompass withinits scope alternative forms of stimulation where such meaning is notprecluded in a particular context.

A current management system implemented in the stimulation (or therapy)output stage of an implantable neurostimulation system is described inwhich each of several electrodes available for use in a stimulationepisode is provided with its own stimulation electrode drive circuit,which can be configured to source or sink current (with respect to thepatient) based on digital control and timing signals that are derivedfrom programmed instructions conveyed through or by another system or byanother subsystem of the neurostimulation system, and a furtherreference electrode is provided to be maintained at a constant referencevoltage, by sinking or sourcing current to balance what is happening atthe stimulation electrodes.

FIG. 1 is a schematic diagram illustrating a current management system100 for a neurostimulation system including stimulation electrodes SE₁110, SE₂ 112, SE₃ 114 through SE_(N) 120 each connectable through itsown stimulation electrode drive circuit, EDC₁ 170, EDC₂ 172, EDC₃ 174through EDC_(N) 180 to a supply voltage V_(S) 190. Throughout operationof the current management system, the reference electrode RE 160 isconfigured to maintain a constant voltage V_(RE) 192 by source andsinking currents through a patient 150 to balance any currents sunk orsourced through the stimulation electrodes SE₁ 110, SE₂ 112, SE₃ 114through SE_(N) 120, as is explained more fully below. The voltage V_(RE)to be maintained at the reference electrode RE 160 might be half thesupply voltage V_(S) 190. (In other variations, the current managementsystem may be configured to maintain the reference electrode RE atground potential, where the electrode drive circuits are powered byvoltages around ground, e.g., by a voltage V_(P) that is above ground,and a voltage V_(N) equal to V_(P) but below ground, such as V_(P)=+8 Vand V_(N)=−8 V.) The programmed instructions determine whether and whenany given switch to connect a stimulation electrode to a patient isclosed during all or a part of a stimulation episode. If a stimulationelectrode is used in a stimulation episode, the stimulation electrodedrive circuit associated with that stimulation electrode will configurethe stimulation electrode to source or sink current at the appropriatetimes according to the parameters of the stimulation episode.

The action of each stimulation electrode drive circuit that isprogrammed to be active during a stimulation episode (“active electrodedrive circuit”) will be controlled by at least one control signal and atleast one clock signal. More specifically, the control signal(s)together with the timing signal(s) will determine, during a givenstimulation episode, whether a particular stimulation electrode issourcing or sinking current during discrete time periods or segments ofthe stimulation episode and, if so, what will be the amplitude of thecurrent. Alternatively, the control signal(s) together with the timingsignal(s) can be used to cause an electrode to look like an open circuit(high impedance), for example, to “turn it off” during a segment orduring a portion of a segment, such as when transitioning betweensourcing and sinking functions (similar to a “break-before-make”condition that might be used with control of a switch). In still othercircumstances, the control and timing signals can be used to cause theelectrode to look like a short circuit (low impedance), for example, toallow any charge that has built up at the electrode-to-tissue interfaceto discharge. Thus, the control and timing signal(s) in operation witheach stimulation electrode's electrode drive circuit can cause eachstimulation electrode to have one of four possible functions during eachsegment of a stimulation episode, namely, a current source with a givenamplitude, a current sink with a given amplitude, a high impedance, or alow impedance.

FIGS. 2A-2C illustrate one example of an interaction between a controlsignal and a timing signal with respect to what is happening at onestimulation electrode SE₁ 110 (i.e., the function the electrode servesbased on the control and timing signals. FIG. 2A is a representation ofa control signal 210 with time on the x-axis for a stimulation electrodeSE₁ 110. Packets of information, as might be delivered over a bus, areindicated as “DATA1” 212, “DATA2” 214, “DATA3” 216 and “DATA4” 218. Eachpacket of information can specify the function the electrode is toperform, as well as certain parameters corresponding to a predeterminedstimulation episode. For example, if DATA1 212 contains information thatwill set the function of the stimulation electrode SE₁ 110 to perform asa current source (i.e., to deliver current to the patient), the DATA1212 packet may also specify the strength of the current (e.g., currentamplitude in mA). A timing signal 220, such as derived from a clock, isshown in FIG. 2B, and the function of the electrode SE₁ 110 relative tothe timing signal 220 is shown in FIG. 2C. Thus, when the timing signal220 is high during the time period T_(CKHIGH) 224, the stimulationelectrode SE₁ 110 will carry out the function specified in the packetDATA1 212, e.g., to source current into the patient at an amplitude of XmA. The current sourcing function of the stimulation electrode SE₁ 110during the first clock high period T_(CKHIGH) 224 is shown in FIG. 2C asthe rectangle 262.

Optionally, it may be desirable for the function of the stimulationelectrode to transition briefly to a high impedance state after onefunction is performed and the next function begins (similar to a“break-before-make” design as might be used in a switching circuit).Such a short high impedance transition can be appreciated with referenceto FIGS. 2B and 2C. More specifically, whenever the clock signaltransitions from a high state to a low state at T_(TRANS) 226, thestimulation electrode SE₁ 110 is configured to have a high impedance(i.e., look like an open circuit) 264 for a short period of timerelative to the time periods in which the stimulation electrode SE₁ 110is performing one of its functions. In one variation, an appropriateduration for this brief “High Z” state between different electrodefunctions may be 200 nS. In practice, the brief high impedance statebetween transitions of an electrode from one function to another may beaccomplished with the timing signals used to enable or disable the flowof current through an electrode. For example, the programming may besuch it causes the flow of current through an electrode to start after ashort delay (e.g., when a sourcing or sinking function for thatelectrode is enabled) but it causes the flow of current to stop withoutdelay (e.g., when a sourcing or sinking function is disabled). This willput an electrode in a high impedance state during a transition betweenfunctions in which current is programmed to flow through the electrode.The short High Z period may be implemented by adjusting the digitaltiming and/or control signals.

Referring still to FIGS. 2A-2C, after the short high impedance period264, the presence of information packet DATA2 214 in the control signal210 causes the stimulation electrode SE₁ 110 to change functions andperform as a current sink 266 (i.e., sinking current out of thepatient). The amplitude, Y mA, of this current is specified in thepacket DATA1 214. The stimulation electrode during the second transitionT_(TRANS) 226 shown in FIG. 2B will continue to sink Y mA of current aslong as the timing signal 220 stays high (during the second time periodT_(CKHIGH) 224 in FIG. 2B). When the timing signal 220 again transitionsto a low state (during the second transition T_(TRANS) 226 shown in FIG.2B), the stimulation electrode SE₁ 110 will be in a high impedance state264 for a brief period before transitioning to the function specified inthe next information packet in the control signal 210, namely, DATA3216. DATA3 216 specifies a short circuit for the stimulation electrodefunction. Thus, following the third brief high impedance state 264 shownin FIG. 2C, the stimulation electrode SE₁ 110 will perform as a shortcircuit for so long as the timing signal remains high (i.e., the thirdtime period T_(CKHIGH) 224 in FIG. 2B. Finally, on the next transitionof the timing signal 220 from high to low (during the third transitionT_(TRANS) 226 shown in FIG. 2B), and after the fourth brief highimpedance state 264 shown in FIG. 2C, the stimulation electrode SE₁ 110will remain in the high impedance state, since that is the function forelectrode that is specified in the last information packet 218 of thecontrol signal 210 shown in FIG. 2A. Thus, the stimulation electrode SE₁110 will look like a high impedance to the rest of the circuit, for solong as the timing signal 220 remains high (during the fourth timeperiod T_(CKHIGH) 224 in FIG. 2B). It will be apparent that astimulation episode as realized at a given one of the stimulationelectrodes may be controlled by different control signals that have moreor less information regarding electrode function than is described inconnection with FIGS. 2A-2C above. Similarly, it will be appreciatedthat a timing signal for a stimulation electrode can be provided withmore or less complicated transitions may be used in connection with agiven stimulation episode.

It will be apparent to one skilled in the art that if a single clocksignal is used for all of the stimulation electrodes that are selectedfor a given stimulation episode, then the electrodes will changefunctions according to what is specified in each electrode's controlsignal at the same times. In other words, the electrodes will functionsynchronously with each other. Of course, each electrode neverthelessmay be performing different functions at different times even when theyshare a single clock signal. For example, if three stimulationelectrodes SE₁ 110, SE₂ 112, and SE₃ 114 are used to deliver stimulationin a given stimulation episode and each of the stimulation electrodes isassociated with its own control signal 210 but with a common timingsignal 220, the control signals may specify different functions for eachof SE₁ 110, SE₂ 112, and SE₃ 114 even though function changes (to theextent any are specified in the control signals) will occur at the sametime. For the first time period when the timing signal 220 is high(T_(CKHIGH) 224), the first stimulation electrode SE₁ 110 might have thefunction of a current source at X₁ mA, the second stimulation electrode5E₂ 112 might have the function of looking like a high impedance, andthe third stimulation electrode SE₃ 114 might have the function of acurrent source at X₂ mA. After the transition to the second time periodstimulation electrode when the timing signal 220 is high (T_(CKHIGH)224), the first stimulation electrode SE₁ 110 might have the function ofa current source at X₃ mA, the second stimulation electrode SE₂ 112 maycontinue to be high impedance, and the third stimulation electrode SE₃114 may change to a current sink at Y₁ mA. Alternatively, and with ashared timing signal 220, some of the stimulation electrodes may have a“break-before-make” high impedance state between transitions from onetype of function (e.g., sourcing) to another (e.g., sinking) and somemay not.

On the other hand, if the stimulation electrodes do not share a clocksignal but rather each electrode is supplied with its own dedicatedclock signal, it would be possible to cause each electrode to operateaccording to different phases within a stimulation episode, for example,one electrode could be switching between current sourcing and currentsinking functions twice as fast as another electrode is switchingbetween sourcing and sinking functions, or one electrode could remainshorted for half as long as another electrode remains high impedance,and so on and so forth. Indeed, it is envisioned that in somevariations, multiple stimulation electrodes may be provided with thesame control signal but different timing signals, so that differentfunctions for different electrodes might be enabled at different times.

At any given instant during a stimulation episode, the referenceelectrode RE will try to source or sink current as necessary (i.e.,based on how each stimulation electrode is functioning) to maintain thevoltage V_(RE) constant. For example, if, at a time t₁, the stimulationelectrode drive circuits for the stimulation electrodes active for thisstimulation episode try to deliver stimulation so that a net current of3 mA will be sourced into the patient, the reference electrode will haveto sink 3 mA from the patient. If there is not sufficient voltagedifference between the reference electrode (V_(RE)) and the positive ornegative supply voltage to allow this to happen, then the programmedcurrent cannot be delivered. (In one variation, the condition where theprogrammed amount of current cannot be sourced or sunk due toinsufficient reservoir voltage is called a “TILT” and is described morefully below). However, if there is sufficient voltage for the referenceelectrode to sink the 3 mA, the amount of current going into the patientwill be balanced by the amount of current going out of the patient, thussatisfying Kirchhoff s current law for the circuit.

Thus, the presence of the reference electrode in the circuit allows eachstimulation electrode to perform its functions independently of what ishappening at every other stimulation electrode. When the stimulationelectrodes are programmed to source current, this means that knownamounts of current can be steered through different stimulationelectrodes at desired times, thus providing the person who isprogramming the neurostimulation system (i.e., the programmer) with theability to deliver stimulation of varying strengths to the locations inthe patient adjacent each electrode.

This is to be contrasted to the situation in which a single currentsource is used to drive a parallel combination of electrodes. In thissituation, due to impedance mismatching among the parallel electrodes(and any lead(s) through which the electrodes are connected to thecurrent generator), the amount of current passing through each electrodecannot be precisely controlled. FIG. 3 illustrates a stimulation outputstage for a neurostimulation system when two or more electrodes areconfigured to source or sink current in parallel. In FIG. 3, there arefour electrodes E_(A) 310, E_(B) 312, E_(C) 314 and E_(D) 316 adjacentthe tissue of a patient (the patient is not represented in FIG. 3). Thefour electrodes are arranged in parallel in a circuit supplied by avoltage V_(SX) 302. An electrode can be selected to source or sinkcurrent by the switches SW_(ASO) 320, SW_(ASK) 322, SW_(BSO) 324,SW_(ASK) 326, SW_(CSO) 328, SW_(CSK) 330, SW_(DSO) 332, SW_(DSK) 334.For example, the first electrode E_(A) 310 can be enabled to sourcecurrent into the patient by closing switch SW_(ASO) 320 to connect thefirst electrode E_(A) 310 to the voltage V_(SX) 302, or to sink currentout of the patient by closing SW_(ASK) 322 to connect the firstelectrode E_(A) 310 to ground. For a particular stimulation episode, thefirst electrode E_(A) 310, the second electrode E_(B) 312, and the thirdelectrode E_(C) 314 might be selected from among the electrodesavailable for stimulation. More particularly, the first and secondelectrodes E_(A) 310 and E_(B) 312 may be selected to source currentinto the patient, for example, because the programmer wants to stimulatethe area adjacent those two electrodes, and the third electrode E_(C)314 may be selected as a return electrode, to sink the current sourcedby the first and second electrodes out of the patient. To accomplishthis, switch SW_(ASO) 320 would be closed to source current through thefirst electrode E_(A) 310, switch SW_(BSO) 324 would be closed to sourcecurrent through the second electrode E_(B) 312 and switch SW_(CSK) 330would be closed to sink the current sourced by the first and secondelectrodes through the third electrode E_(C) 314. The programmer mighthave in mind that, since the first and second electrodes E_(A) 310 andE_(B) 312 are connected in parallel, equal amounts of current will besourced through each electrode. However, if the impedances of the firstand second electrodes are not matched, then the programmer will not bein a position to know exactly how much of the total amount of currentthat is programmed to be sourced will be sourced through each of thefirst and second electrodes. For example, if the total amount of currentthat is programmed to be sourced is 1.5 mA, and the impedance of thefirst electrode E_(A) 310 is half as much as the impedance of the secondelectrode E_(B) 312, the first electrode will carry twice as muchcurrent as the second electrode, or the first electrode will carry 1.0mA and the second electrode will carry 0.5 mA. (As will be appreciatedby those with skill in the art, the programmer's decision of howstrongly to stimulation through the electrodes selected for stimulationalso will be informed, in part, by the charge density at the electrodeswhich is dependent on the electrode surface area.) In any event, and asa practical matter, the programmer will not know the exact impedances atthe stimulation electrodes at any given moment in time, and theimpedances will change with time, so the opposition to the currentflowing through a given electrode as compared to other stimulationelectrode cannot be predetermined or predicted for any particularstimulation episode.

Returning now to the instant current management system for a therapyoutput stage of a neurostimulation system, the reference electrodepreferably is configured to have a surface area that is sufficient tokeep whatever currents are flowing through it at a low enough chargedensity so that the functioning of the reference electrode is not afactor the programmer must take into consideration when determining thestrength of the stimulation to be delivered at any given stimulationelectrode at any given phase.

With reference now to FIG. 4, a perspective view of one type ofimplantable neurostimulation system 400 is illustrated. Theneurostimulator 410 is situated in a ferrule 420 that is seated in anopening formed in the patient's cranium (as by a craniotomy) andattached to the cranium with fasteners 422 using bone screws or thelike. The electrodes available for use in a stimulation episode arelocated near the distal end of each of two brain leads 430, 440(electrodes not shown in FIG. 4). Each of the leads 430, 440 isintroduced to the patient's brain through a burr hole 450.

Two different types of brain leads are indicated in FIG. 4. Lead 430 isreferred to as a depth lead or deep brain lead, because the ringelectrodes (not shown) at the distal end 432 thereof are implanted in apatient's brain. Lead 440 is referred to as a cortical strip leadbecause the disc-shaped electrodes (not shown) disposed on a strip 442on distal end 444 thereof are usually implanted adjacent a surface ofthe brain as opposed to in the brain, such as on a surface of the brainunder the dura mater. Electrode-bearing deep brain leads and corticalstrip leads are described, for example, in U.S. Pat. No. 7,146,222 toBoling for “Reinforced Sensing and Stimulation Leads and Use inDetection Systems,” issued Dec. 5, 2006.

The proximal portions of each brain lead 430, 440 extend over thecranium and each brain lead is connected near the proximalmost portionthereof to a lead connector 460 attached to the neurostimulator 410. Astrain relief 462 is provided at the point where the distal ends of thebrain leads 430, 440 are connected to the lead connector 460. The leadconnector 460 puts the electrodes at the end of each brain lead 430, 440in operable communication with the systems and subsystems of theneurostimulator 410 (e.g., a programmable therapy subsystem associatedwith a pulse generator), which are contained with a housing or devicecase 470. (Neurostimulation systems, including the components andsubsystems of the neurostimulator are described in, for example, U.S.Pat. No. 6,016,449 to Fischell et al. for “System for TreatingNeurological Disorders,” issued Jan. 18, 2000; and U.S. Pat. No.6,810,285 to Pless et al. for “Seizure Sensing and Detection Using andImplantable Device,” issued Oct. 26, 2004. U.S. Pat. No. 6,690,974 toArcher et al. for “Stimulation Signal Generator for an ImplantableDevice” issued Feb. 10, 2004 includes a description of an outputstimulation stage for a neurostimulation system.)

The device case 470 and the ferrule 420, among other components of theneurostimulation system, are formed of a biocompatible metal, such astitanium, and thus constitute electrically conductive surfaces which canbe used alone or together as a reference electrode RE 160. Theconductive area of the device case typically is much larger than theconductive area of each of the stimulation electrodes, so the currentdensity at the device case will be much lower than any current densitiesat a stimulation electrode-to-tissue interface. For example, in theneurostimulation system currently under investigation by NeuroPace, Inc.under the name “RNS SYSTEM,” the surface area of the housing (or devicecase or “can”) of the implantable neurostimulator is on the order of 30cm.sup.2. The RNS SYSTEM can be configured with either depth brain leadsor cortical strip leads, and each electrode on either type of lead has asurface area on the order of 0.8 cm.sup.2.

The independent control of the stimulation electrodes characteristic ofthe current management system 100 will be further described withreference to FIGS. 5-7.

FIG. 5 illustrates stimulation electrode drive circuits EDC₁ 170, EDC₂172 through EDC_(N) 180, operably associated with stimulation electrodesSE₁ 110, SE₂ 112 through SE_(N) 120 and reference electrode RE 160.Power from a battery P_(BATTERY) 510 (for example a primary cell(non-rechargeable) battery or a secondary (rechargeable) batteryincluded as a power source in the implantable neurostimulator (see,e.g., the neurostimulator 410 of FIG. 4) is boosted with first andsecond boost converters 512, 514 to create the voltages between whichthe stimulation electrode drive circuits will operate. As will beappreciated by one skilled in the art, the boost converter is a DC-to-DCconverter with an output voltage that is greater than the sourcevoltage, which in one variation is from a voltage that is referenced toground and derived from the positive pin of a battery with a voltage ofapproximately 3V. The first boost converter 512 is configured to createa top rail voltage V_(TOPRAIL) 520 and the second boost converter 514,is configured to create a bottom rail voltage V_(BOTTOMRAIL) 522. Thevoltages V_(TOPRAIL) 520 and V_(BOTTOMRAIL) 522 may be above and belowground potential, respectively, for example, in one variation,V_(TOPRAIL) 520=V_(p)=+8.0 V and V_(BOTTOMRAIL) 522=V_(N)=−0.8 V. Inother variations, V_(TOPRAIL) 520 may be above ground potential andV_(BOTTOMRAIL) 522 may be at ground potential for example, where thetechnology used in implementing the circuit necessitates the use ofvoltages that are at or above ground. An example of a current managementsystem using all voltages that are at ground potential or higher isdescribed with references to FIG. 9.

With continued reference to FIG. 5, each of V_(TOPRAIL) 520 andV_(BOTTOMRAIL) 522 are adjustable between a maximum voltage (e.g.,V_(TOPRAIL) 520=+8 V, V_(BOTTOMRAIL) 522=−8 V) to source or sink currentthrough the stimulation electrodes SE₁ 110 through SE_(N) 120 and RE160. Each of the stimulation electrodes SE₁ 110, SE₂ 112 through SE_(N)120 is associated with its own stimulation electrode drive circuit EDC₁170, EDC₂ 172 through EDC_(N) 180. Each stimulation electrode drivecircuit has a current sourcing portion and a current sink portion, i.e.,current source portion CSOURCE₁ 540 and current source portion CSINK₁560 of electrode drive circuit EDC₁ 170 are associated with stimulationelectrode SE₁ 110, CSOURCE₂ 542 and current source portion CSINK₂ 562 ofelectrode drive circuit EDC₂ 172 are associated with stimulationelectrode SE₂ 112, and so on, such that if there are N stimulationelectrodes, CSOURCE_(N) 550 and current source portion CSINK_(N) 570 ofelectrode drive circuit EDC_(N) 180 are associated with stimulationelectrode SE_(N) 120. The current source portions CSOURCE₁ 540, CSOURCE₂542 through CSOURCE_(N) 550 have a first logic control 580 that isdistinct from a second logic control 590 that is provided for CSINK₁560, CSINK₂ 562 through CSINK_(N) 570 so that, for any given stimulationelectrode SE₁ 110, SE₂ 112 through SE_(N) 120 the current managementsystem 100 cannot try to source and sink current through a stimulationelectrode at the same time.

In this configuration, the current that any one stimulation electrodeSE₁ 110, SE₂ 112 through SE_(N) 120 is able to deliver is unaffected bythe impedance at any of the other stimulation electrodes SE₁ 110, SE₂112 through SE_(N) 120, provided that the voltages V_(TOPRAIL) 520 andV_(BOTTOMRAIL) 522 are sufficient to drive whatever amount of currentthe electrode is programmed to deliver. In other words, because eachstimulation electrode is associated with an independent electrode drivecircuit rather than having one current source/sink circuit associatedwith all of the electrodes, each electrode will source or sink theprecise amount of current it is programmed to source or sink, regardlessof what is happening at any other electrode. In order to maintain thevoltage V_(RE) at the reference electrode RE 160 constant, the referenceelectrode will sink or source current as necessary to keep the totalamount of current balanced (i.e., so that the net current going into thepatient is equal to the net current coming out of the patient at anygiven instant during a stimulation episode).

Of course, a programmed amount of current may not be deliverable througha given stimulation electrode when the product of the impedance at theelectrode and the programmed current is greater the voltage available tosupply the applicable current source or sink (e.g. the differencebetween one of the rail voltages V_(TOPRAIL) 520 or V_(BOTTOMRAIL) 522and the reference electrode voltage V_(RE) 192). This condition, wherethe stimulation reservoir voltage is insufficient to support theprogrammed stimulation current is called a “TILT.” As will be discussedin more detail below, one variation of a current management system hasfeatures that allow the system to automatically adjust itself toincrease the reservoir voltage so that the programmed current can besupported.

An example of operation of the current management system will now bedescribed with reference to FIG. 6A-6E, all of which represent time onthe x-axis and current amplitude on the y-axis. A stimulation episode600 is depicted in FIG. 6A, as a biphasic pulsatile waveform 602 formedfrom a series of biphasic pulses of equal amplitude (a pulse traincomprising two instances the same positive and negative pulses (fourpulses altogether) is shown in FIG. 6A) over the time period T_(X) 604.One of the two positive/negative combination comprises a first pulse 620with an amplitude of 2 mA lasting for a time T₁ 606 and a second pulse622 with an equal and opposite amplitude of −2 mA lasting for a time T₂608. After a time T₃ 610 in which there is no current stimulationscheduled, i.e., the stimulation signal portion 624 of the stimulationepisode 600 is intended to be inactive (i.e., zero), the second of thetwo positive/negative combinations comprises a third pulse 626 lastingfor a time T₄ 612 and a fourth pulse 628 lasting for a time T₅ 614.

For this particular stimulation episode 600, a programmer has determinedthat it would be desirable to stimulate the area in the vicinity of thefirst stimulation electrode SE₁ 110 most strongly, with pulses having anamplitude of +−1.5 mA over the time period T_(X) 604 (as shown in FIG.6B), and to stimulate the area in the vicinity of the second stimulationelectrode SE₂ 112 less strongly, with pulses having an amplitude of+−0.5 mA. A third stimulation electrode SE₃ 114 is configured as areturn electrode (e.g., in a bipolar configuration with one or both ofthe first and second stimulation electrodes 110, 112). However, thethird stimulation electrode SE₃ 114 is programmed to only source andsink current having an amplitude of +−1 mA over the time period T_(X).So the reference electrode RE 160 is left with having to source and sinkcurrent having an amplitude of +−1 mA in order to balance the netcurrent going into the patient (i.e., +−1.5 mA from the firststimulation electrode SE₁ 110 and +−0.5 mA from the second stimulationelectrode SE₂ 112) with the net current returning through the patient toground (i.e., the current being sunk by the patient, +−1.0 mA throughthe third stimulation electrode SE₃ 114 and +−1.0 mA through thereference electrode RE 160).

More specifically, and referring to FIGS. 6B and 6C, at T₁ 606, thestimulation electrode SE₁ 110 delivers a positive 1.5 mA pulse 640 tothe patient (i.e., sources 1 mA) and the stimulation electrode SE₂ 112delivers a positive 0.5 mA pulse 660 to the patient, totaling the +2 mAcalled for at T₁ in the stimulation episode 600. Also in the first timeperiod at time T₁ 606, the stimulation electrode SE₃ 114 sinks 1.0 mAthrough the patient (indicated by the −1.0 mA pulse 670 in FIG. 6D).Referring to FIG. 6D, the reference electrode RE 160 sinks 1.0 mA (asrepresented by the −1.0 mA signal 680) at time T₁ 606, in order to keepthe voltage at the reference electrode, V_(RE) 192, constant. Referringto the same FIGS. 6A-6D, at time T₂ 608, the stimulation episode 600calls for a −2.0 mA pulse 622, and the stimulation electrode SE₁ 110 andthe stimulation electrode SE₂ 112 are programmed to sink a total of 2.0mA (i.e., −1.5 mA through stimulation electrode SE₁ 110 at signalportion 642 and −0.5 mA through stimulation electrode SE₂ 112 at signalportion 662). But the third stimulation electrode SE₃ 114 is onlyprogrammed to source 1.0 mA at time T₂ 608 (see the third stimulationelectrode signal portion 672), so the reference electrode RE 160 has tomake up the difference and source another 1.0 mA at time T₂ 608 (see thereference electrode signal portion 682).

During the next period 624 in the stimulation episode 600 at time T₃610, all of the stimulation electrodes are effectively off (no currentis flowing through any of them) (at time T₃ 610, see the signal portion644 for the first stimulation electrode SE1 110 in FIG. 6B, the signalportion 664 for the stimulation electrode SE₂ 112 in FIG. 6C, and thesignal portion 674 for the third stimulation electrode SE₃ 114 in FIG.6C), so the reference electrode does not need to source or sink anycurrent (at time T₃ 610, see the signal portion 684 for the referenceelectrode RE 160).

The second of the two sets of biphasic pulses occurs in the stimulationepisode 600 at times T₄ 612 with the positive 2.0 mA pulse 626 and T₅614 with the negative 2.0 mA pulse 628. The operation of the threestimulation electrodes and the reference electrodes is repeated from thefirst and second time period T₁ 606 and T₂ 608 for the fourth and fifthtime periods T₄ 612 and T₅ 614 based on the stimulation episode 600 andthe stimulation electrode programming. That is, at time period T₄ 612,the stimulation electrode SE₁ 110 sources 1.5 mA (see the signal portion646 in FIG. 6B), the stimulation electrode SE₂ 112 sources 0.5 mA (seethe signal portion 666 in FIG. 6C), the stimulation electrode SE₃ 114sinks 1.0 mA (see the signal portion 676 in FIG. 6D), and the referenceelectrode 160 sinks 1.0 mA (see the signal portion 686 in FIG. 6E) tomaintain the voltage V_(RE) 192 at the reference electrode constant.Then, at time period T₅ 614, the stimulation electrode SE₁ 110 sinks 1.5mA (see the signal portion 648 in FIG. 6B), the stimulation electrodeSE₂ 112 sinks 0.5 mA (see the signal portion 668 in FIG. 6C), thestimulation electrode SE₃ 114 sources 1.0 mA (see the signal portion 678in FIG. 6D), and the reference electrode 160 sinks 1.0 mA (see thesignal portion 688 in FIG. 6E).

The foregoing example is a relatively simple one (e.g., nobreak-before-make conditions are illustrated) and the three stimulationelectrodes are all programmed to change function at the same times. Itwill be appreciated that the reference electrode RE 160 can continuouslybalance the current being sourced or sunk relative to the patient whenthe stimulation episodes and programming are far more complex, such aswhen different control signals for different stimulation electrodes arederived from a given stimulation episode, or the timing of when eachstimulation electrode changes function is different for different onesof the stimulation electrodes within a given stimulation episode, orwhen there is more than one stimulation episode to be delivered througha given set of stimulation electrodes at the same time, etc.

Some additional examples include: (1) programming a first stimulationelectrode SE1 to source 1 mA and a second stimulation electrode SE₂ tosource 1 mA during a first time T₁ of a stimulation episode, where nostimulation electrodes are programmed to sink current at time T₁: inthis case, the reference electrode RE will sink 2 mA; (2) programming afirst stimulation electrode SE₁ to source 1 mA, a second stimulationelectrode SE₂ to source 1 mA, and a third stimulation electrode SE₃ tosink 1 mA during a first time T₁ of a stimulation episode, such that thereference electrode RE will sink 1 mA; (3) programming a firststimulation electrode SE₁ to source 1.5 mA for the first half of a firsttime period T₁ and to sink 0.5 mA for the second half of the first timeperiod T₁, programming a second stimulation electrode SE₂ to sink 1.0 mAfor all of the first time period T₁: in this case, the referenceelectrode will sink 0.5 mA for the first half of the first time periodT₁ and will source 0.5 mA for the second half of the first time periodT₁; and (4) programming each of three stimulation electrodes to source 2mA and programming no stimulation electrode to sink any current, thusleaving the reference electrode to sink all 6 mA.

FIG. 7 illustrates the action at the reference electrode 160 in anexample that is slightly more complex than the example illustrated inFIGS. 6A-6E. In FIG. 7, the graphs 710 and 720 correspond to thebehavior of the first and second stimulation electrodes SE₁ 110 and SE₂112 programmed based on the parameters of a first stimulation episode,stimulation episode “Z₁,” corresponding to the top graph 710 of FIG. 7.The graphs 750 and 760 correspond to the third and fourth stimulationelectrodes SE₃ 114 and SE₄ 116 programmed based on the parameters of asecond stimulation episode, stimulation episode “Z₂” corresponding tothe bottom graph 740 of FIG. 7. The action at the reference electrode RE160 corresponds to the middle graph 780 in FIG. 7. Both the firststimulation episode Z₁ and the second stimulation episode Z₂ arecomposed of sets of biphasic current pulses separated by an interval inwhich no current is being sourced or sunk (intervals T₃ and T₆ in FIG.7). However, the pulses in the second stimulation episode Z₂ aredelivered twice as fast as the pulses in the first stimulation episodeZ₁.

Referring now to FIG. 8, one way of implementing an electrode drivecircuit for each of the electrodes available for use in stimulation in acurrent management system will now be described. An electrode drivecircuit 800 is provided for stimulation electrode SE₁ 110 having acurrent sink portion 804 and a current source portion 806. The currentsink portion 804 is provided with a constant current reference I_(REF1)820 and the current source portion 804 is provided with a constantcurrent reference I_(REF2) 860. As explained above, when a givenstimulation electrode SE₁ 110 through SE_(N) 120 is being used in thecourse of a stimulation episode 600, the current sink portion 804 andthe current source portion 806 cannot be operating at the same time.

Operation of the current sink portion 804 can be explained as follows:In this variation, the supply voltage V_(S) 190 is a regulated supplythat is referenced to ground and derived from a positive pin of abattery. Thus, the first constant reference current, I_(REFI) 820 issubstantially independent of voltage supply variations. The firstconstant reference current I_(REFI) 820 flows through a first fieldeffect transistor (nFET) M1 830 that is diode-connected. A second nFETtransistor M2 840 is configured to mirror the current in the firsttransistor M1 830. The sources of the first and second nFET transistorsM1 830 and M2 840 are at a potential corresponding to V BOTTOMRAIL,which in one variation is a negative voltage V_(N), e.g., −8.0 V). Afirst operational amplifier 850 modulates the gate of a third nFETtransistor M3 858 so that the voltage at the drains of the first andsecond nFET transistors M1 830 and M2 840 are maintained as equal. Thatis, a first input 852 to the first operational amplifier 850 is thevoltage at the drain of the first nFET transistor M1 830 and a secondinput 854 to the first operational amplifier 850 is the voltage at thedrain of the second (mirror) nFET transistor M2 840. A first operationalamplifier output 856 is input to the gate of the third nFET transistorM3 858, and the drain of the third transistor nFET M3 858 is connectedto the first stimulation electrode SE₁ 110. Thus, as the voltage at thefirst stimulation electrode SE₁ 110 varies, the first operationalamplifier 850 will modulate the gate of the third nFET transistor M3 858so that the drain voltages of the first and second nFET transistors M1830 and M2 840 are equal. This configuration will keep the currentthrough the mirroring transistor, i.e., the second nFET transistor M2840 constant over a wide range of voltages at the first stimulationelectrode SE₁ 110.

Operation of the current source portion 806 is substantially similar tothat of the current sink portion 804 as described above, although pFETtransistors are used instead of nFET transistors. More specifically, thecurrent passing through a fourth diode-connected pFET transistor M4 870corresponds to a second reference current I_(REF2) 860. A fifth pFETtransistor M5 872 is configured to mirror the current in the fourth pFETtransistor M4 870. The sources of the fourth and fifth pFET transistorsM4 870 and M5 872 are tied to V_(TOPRAIL) 520, which in one variation isa positive voltage V_(P), such as +8 V. A second operational amplifier880 modulates the gate of a sixth pFET transistor M6 888 so that thevoltage at the drains of the fourth and fifth pFET transistors M4 870and M5 872 are maintained as equal. That is, a first input 882 to thesecond operational amplifier 880 is the voltage at the drain of thefirst pFET transistor M4 870 and a second input 884 to the secondoperational amplifier 880 is the voltage at the drain of the second(mirror) pFET transistor M5 872. A second operational amplifier output886 is input to the gate of the third pFET transistor M6 888, and thedrain of the third pFET transistor M6 888 is connected to the firststimulation electrode SE₁ 110. Thus, as the voltage at the firststimulation electrode SE₁ 110 varies, the second operational amplifier880 will modulate the gate of the third pFET transistor M6 888 so thatthe drain voltages of the first and second pFET transistors M4 870 andM5 872 are equal. This configuration will keep the current through themirroring transistor, i.e., the second pFET transistor M5 872 constantover a wide range of voltages at the first stimulation electrode SE₁110.

It will be apparent to one with skill in the art that the current to besourced or sunk by a stimulation electrode may be made programmable in avariety of different ways. For example, in one variation, the range ofpossible currents that can flow through either a current sink portion804 or a current source portion 806 of an stimulation electrode drivecircuit can be increased by implementing the mirror transistors (e.g.,the second transistor M2 840 in current sink portion 804 of electrodedrive circuit 800 in FIG. 8 and the fifth transistor M5 872 in currentsink portion 806 of electrode drive circuit 800) with a number ofdiscrete transistors connected in parallel to control the current gainof the relevant circuit portion. In other words, and again withreference to FIG. 8, if the mirror transistor M2 840 in the current sinkportion 804 is the same size as the diode-connected first transistor M1830, then the current sunk through the first stimulation electrode SE₁110 will be substantially equal to the first reference current I_(IREF1)820. If the mirror transistor M2 840 is instead implemented with, forexample, 100 transistors in parallel and each of these 100 transistorsis the same size as the first diode-connected transistor M1 830, thenthe current through the first stimulation electrode will be 100 timesthe value of the first reference current I_(REF1) 820.

In still other variations, the reference currents for the current sinkand current source portions of an electrode drive circuit for astimulation electrode are programmable between a range of values basedon programming instructions processed by another part of a currentmanagement system 100 (or another part of a neurostimulation system incommunication with a current management system 100). Since the currentflowing into or out of the first stimulation electrode is proportionalto one of the reference currents (e.g., to the first reference currentI_(REF1) 820 if the current sink portion 804 of an electrode drivecircuit 800 is engaged, and to the second reference current I_(REF2) 860if the current source portion 806 of an electrode drive circuit 800 isengaged), then making the reference current programmable makes thecurrent that is sunk or sourced through the stimulation electrodeprogrammable.

In other variations of the current management system, features may beincluded to optimize power consumption. By way of one example, a featureis included one or more electrode drive circuits, such as the electrodedrive circuit 800 shown in FIG. 8, that allows the operationalamplifiers 850, 880 in each of the current sink portion 804 and thecurrent source portion 806 to be selectively controlled by an enablesignal, for example, an enable input. This would allow the current flowthrough the third transistor M3 858 and the sixth transistor M6 888 tobe turned on and off via control logic that causes the enable signal tochange (e.g., go high and low). Since minimizing power consumption in animplantable device is often important (e.g., to avoid prematurelydraining a battery that supplies power for the implantable system), andthe operational amplifiers require power to function, being able to turnoff the operational amplifiers when they are not being used in sourcingor sinking current would be in the interest of power conservation.

In still other variations of the current management system, features maybe included that beneficially can be included to limit the current thatcan flow through a stimulation electrode notwithstanding the programminginstructions and/or increase the current that can be delivered through astimulation electrode when the voltages supplying the stimulationelectrode drive circuits are insufficient to source or sink the amountof current that corresponds to the programming instructions. Forexample, the current management system 100 can be configured toestablish a “TILT” condition whenever the system is unable to source orsink the amount of current it is programmed to source or sink through agiven stimulation electrode. The amount of current programmed to flowwill exceed the amount of current that can be caused to flow through astimulation electrode when the product of the resistance to current flowat the electrode (which may include the impedance of the lead in whichthe electrode is disposed) and the amount of current programmed to flowis greater than available voltage for stimulation (“stimulationreservoir voltage”). In the case of the current sink portion 804 of theelectrode drive circuit 800 of FIG. 8, the stimulation reservoir voltagemay be equivalent to the voltage V_(BOTTOMRAIL) which in turn may beequal to a value V_(N).

When a TILT condition occurs, it may be recognized as the condition whenan operational amplifier in a given stimulation electrode drive circuit(e.g., the first operational amplifier 850 of the current sink portion804 of the electrode drive circuit 800 of FIG. 8) goes to its mostpositive limit (i.e., its positive rail). One way of recognizing a TILTcondition may be to configure a threshold detector so that it compares(e.g., with a comparator) the output of an operational amplifier in acurrent sink portion or a current source portion of a stimulationelectrode drive circuit to a reference voltage.

Upon occurrence of a TILT, the current management system 100 can beconfigured, for example, to do any of the following: (1) prevent anycurrent to flow notwithstanding the programmed instructions; (2)automatically downwardly adjust the amount of current that is programmedto be sunk or sourced to an amount that can be delivered given theexisting stimulation reservoir voltage; or (3) automatically upwardlyadjust the stimulation reservoir voltage (e.g., if the stimulationreservoir voltage at issue is V_(P), increase its value about groundpotential, and if the stimulation reservoir voltage at issue is V_(N),increase its value below ground potential), so that the originallyprogrammed amount of current can flow through the stimulation electrode.

With regard to option (3), above, it will be appreciated that adjustingthe reservoir voltage so that it not substantially greater thannecessary at any given time to deliver a programmed amount of currentwill contribute to the efficiency with which the implantableneurostimulator (see, e.g., neurostimulator 410 in FIG. 4) uses power.For example, in one variation, the two voltage rails V_(TOPRAIL) 520 andV_(BOTTOMRAIL) 522 between which a stimulation electrode drive circuitis configured to operate might be programmed at initial time T₀ to be +3V and −3V, respectively, where the battery that supplies theneurostimulation system is a 3 V battery. If at a later time T1 thestimulation reservoir voltage is insufficient to support the programmedamount of current through a stimulation electrode (e.g., a TILTcondition occurs), then the current management system 100 can beconfigured to automatically upwardly adjust the relevant stimulationreservoir voltage (e.g., V_(TOPRAIL) 520 or V_(BOTTOMRAIL) 522) up tothe minimum voltage value that will support the programmed current. Thevalues of the relevant stimulation reservoir voltages can beautomatically adjusted throughout the life of the implantableneurostimulator (see, e.g., neurostimulator 410 in FIG. 4) in order tomaintain optimum efficiency for the stimulation output stage.

FIG. 9 is a schematic diagram of a variation of a current managementsystem 100 for an implantable neurostimulator 12 in which the referenceelectrode RE 160 is maintained at a constant voltage in a range betweenground and the positive voltage Vp (as contrasted with, for example, acurrent management system 100 that operates between positive andnegative rails). In FIG. 9, the stimulation electrode drive circuitsEDC₁ 170, EDC₂ 172 through EDC_(N) 180 are configurable to sink orsource current within the limits of stimulation reservoir voltagescorresponding to a voltage V_(BOTTOMRAIL) 910 and a voltage V_(TOPRAIL)920, and the reference electrode RE 160 is maintained at a constantpotential V_(MIDRAIL) 930 that is somewhere between the values ofV_(BOTTOMRAIL) 910 and V_(TOPRAIL) 920. For example, an implantableneurostimulator (see, e.g., the neurostimulator 410 of FIG. 4) may havea power supply comprising a 3 V battery. One or more boost converterscan be used to increase the voltage from the battery to create aV_(TOPRAIL) 920=16 V and a V_(BOTTOMRAIL) 910=0 V. The midrail voltagemay be set at a point approximately half way between the two rails, orat V_(MIDRAIL) 930=8 V. The current sink portions (not shown in FIG. 9)of the electrode drive circuits EDC₁ 170, EDC₂ 172 through EDC_(N) 180may be configured to operate between the boundaries of 0 V and +8 V, andthe current source portions (also not shown in FIG. 9) of the electrodedrive circuits EDC₁ 170, EDC₂ 172 through EDC_(N) 180 may be configuredto operate between the boundaries of +8 V and +16 V.

Referring still to FIG. 9, a current management system 100 may beprovided with a reference electrode drive circuit EDC_(RE) 940 whichcomprises a digital-to-analog converter (DAC) 942 and an referenceelectrode operational amplifier 944. Digital signal(s) 980 input to theDAC 942 determine a constant voltage signal (corresponding to or derivedfrom a programmed value) from which the DAC 942 sets a top rail voltageV_(TOPRAIL) 920 and a voltage reference 950. An enable signal 952 forthe reference electrode operational amplifier 944 also is derived from adigital signal. The reference electrode operational amplifier 944 setsand maintains a selected voltage on the reference electrode 160 inresponse to the voltage reference 950 provided by the DAC 942 and theenable signal 952.

The DAC 942 also provides a voltage V_(TOPRAIL) 920 that is the upperrail to a bias circuit 970. The bias circuit 970 receives digitalcontrol 976 and timing (e.g., clock) signals 978 as inputs(corresponding to programmed values) and produces an analog output 972and a digital output 974. The analog output 972 corresponds to theinformation about the current for a particular sourcing or sinkingsegment of a stimulation episode for a particular stimulation electrode.The digital output 974 enables the corresponding stimulation electrodeto perform its designated function. For example, the digital controlinput 976 and the timing input 978 to the bias circuit 970 may containprogrammed instructions to select first stimulation electrode SE₁ 110 tosource 1.5 mA at a first time period T₁ in a stimulation episode. Thebias circuit 970 is configured to generate an analog signal 972 for thefirst stimulation electrode SE₁ 110 that establishes a reference currentfor the electrode of 1.5 mA. The digital output 974 comprises an enablesignal for the first stimulation electrode SE₁ 110 so that, for example,the operational amplifier of the current source portion of thestimulation electrode drive circuit EDC₁ 170 is enabled to allow the 1.5mA to flow through the first stimulation electrode SE₁ 110 for the firsttime period T₁. Although in FIG. 9 only one bias circuit 970 is shownassociated with the stimulation electrode drive circuits EDC₁ 170, EDC₂172 through EDC_(N) 180, it will be appreciated that a dedicated biascircuit may be provided as a feature of each stimulation electrode drivecircuit. Each bias circuit 970 would supply an analog signal 972 to itsassociated stimulation electrode drive circuit(s) EDC₁ 170, EDC₂ 172through EDC_(N) 180 as well as an enable signal 974.

Thus, it will be appreciated that the digital signals which control theoperation of the stimulation electrode drive circuits (e.g., through oneor more bias circuits) convey programmed instructions such as whichfunction an electrode available for stimulation will perform at whichtime during a given stimulation episode (e.g., current source, currentsink, high impedance or short circuit to ground), and what the amplitudeof the current sourced or sunk by an electrode will be when it isconfigured to function as a source or sink. It further will beappreciated that the digital signals will have the effect of selectingwhich of the electrodes available for stimulation to use in a givenstimulation episode. That is, the digital signal(s) enable the currentsourcing portion of the electrode drive circuit EDC₁ 170 for stimulationelectrode SE₁ at time T₁ to source 1 mA of current into the patient,then the stimulation electrode SE₁ 110 has thereby been selected for usein the stimulation episode, at least during time T₁.

With reference again to FIG. 9, each of the stimulation electrode drivecircuits EDC₂ 172, EDC₃ 174 through EDC_(N) 180 is configured togenerate a signal representative of a TILT condition, i.e., TILT signal990 for EDC₁ 170, TILT signal 992 for EDC₂ 172, and TILT signal 998 forEDC_(N) 180, whenever the conditions prerequisite to a TILT conditionfor a given current sink portion or a current source portion for astimulation electrode drive circuit exists.

It should be noted that as illustrated and described herein, each of thevarious components of the variations of the current management system100 described is not necessarily a single physical or functional elementthat can be adequately represented in any illustration, and thatphysical or functional elements may be combined in various ways for thesame or similar effect. For example, and not by way of limitation, afunction described herein as being performed by hardware may beperformed by software or a combination of software and hardware.

In some embodiments, the neurostimulation system (see, e.g., theneurostimulation system 400 of FIG. 4) with which the current managementsystem 100 is used for a stimulation output stage in accordance with oneor more of the foregoing variations can be used in combination withother types of systems for delivering stimulation to a patient. Forexample, the electrodes available for use in a stimulation episode thatcurrent flow through the electrodes could be disabled for current flow(for example, by programming the electrodes to go to a High Z state, andthen the electrodes could be made available for use by anotherstimulation output stage, such as one that delivers voltage stimulationrather than current stimulation.

In still other embodiments, the neurostimulation system with which thecurrent management system 100 is used for a stimulation output stage inaccordance with one or more of the foregoing variations can be used incombination with a system that is configured to sense a physiologicalvariable or variables from the patient. For example, one or more of theelectrodes available for stimulation that are associated with a currentmanagement system 100 can alternatively be used to sense a signal from apatient, such as an electroencephalographic signal (or “EEG”). Toaccomplish this, the current management system 100 might be programmedto set the function of a stimulation electrode to high impedance, duringwhich time the electrode can be used to sense a voltage differential,for example, in a field potential measurement to acquire an EEG signal.It will be apparent that in the context of a neurostimulation systemthat has both stimulation and sensing and/or signal detection and/orsignal recording capabilities, the current management system 100 can beprogrammed so that some of the electrodes available for stimulation areused to source or sink current relative to the patient, and other of theelectrodes available for stimulation can instead be configured forsensing and/or detection and/or recording during a stimulation episodeor between stimulation episodes or both.

While particular embodiments and applications of the present inventionhave been illustrated and described herein, it is to be understood thatthe invention is not limited to the precise construction and componentsdisclosed herein and that various modifications, changes, and variationsmay be made in the arrangement, operation, and details of the methodsand apparatuses of the present invention without departing from thespirit and scope of the invention as it is defined in the appendedclaims.

What is claimed is:
 1. A neurostimulation system comprising: a reference electrode; a plurality of stimulation electrodes; a stimulation electrode drive circuit for each of the plurality of stimulation electrodes, each stimulation electrode drive circuit configured to receive a digital output and an analog output, and to: 1) in response to receipt of a first digital output and a first analog output, source current through its associated stimulation electrode at an amperage value determined by the first analog output, and 2) in response to receipt of a second digital output and a second analog output, sink current through its associated stimulation electrode at an amperage value determined by the second analog output; and a reference electrode drive circuit adapted to maintain the reference electrode at a constant potential when any or all of the stimulation electrodes are sourcing or sinking current.
 2. The neurostimulation system of claim 1, wherein each stimulation electrode drive circuit is configured to source current through its associated stimulation electrode for a first period of time determined by the first digital output.
 3. The neurostimulation system of claim 1, wherein each stimulation electrode drive circuit is configured to sink current through its associated stimulation electrode for a second period of time determined by the second digital output
 4. The neurostimulation system of claim 1, wherein each stimulation electrode drive circuit is further configured to, in response to receipt of a third digital output, cause its associated stimulation electrode to have a high impedance.
 5. The neurostimulation system of claim 4, wherein each stimulation electrode drive circuit is configured to maintain its associated stimulation electrode at the high impedance for a period of time determined by the third digital output.
 6. The neurostimulation system of claim 1, wherein each stimulation electrode drive circuit is further configured to, in response to receipt of a fourth digital output, short its associated stimulation electrode.
 7. The neurostimulation system of claim 6, wherein each stimulation electrode drive circuit is configured to short its associated stimulation electrode for a period of time determined by the fourth digital output.
 8. The neurostimulation system of claim 1, wherein the constant potential of the reference electrode is such that the reference electrode will sink or source an amount of current equal to the total amount of current being sourced or sunk through the stimulation electrodes at any given time in a stimulation episode.
 9. The neurostimulation system of claim 1, wherein each stimulation electrode is capable of sourcing and sinking current independently of any current being sourced or sunk through any other of the stimulation electrodes.
 10. The neurostimulation system of claim 1 wherein each stimulation electrode drive circuit has a current sink portion and a current source portion, and at least one feature that prevents the current sink portion from being enabled at the same time as the current source portion.
 11. The neurostimulation system of claim 1 wherein each stimulation electrode drive circuit has a bias circuit to which a digital control signal and a timing signal are input and from which the digital output and the analog output are derived.
 12. The neurostimulation system of claim 1 wherein each stimulation electrode drive circuit establishes a reference current for its associated stimulation electrode with a current mirror circuit.
 13. A method of managing current for a neurostimulation system having a reference electrode and a plurality of stimulation electrodes, the method comprising: causing each of the plurality of stimulation electrodes, through individual programmed instructions comprising a digital output and an analog output, to: 1) in response to receipt of a first digital output and a first analog output, source current at a reference current having an amplitude determined by the first analog output, and 2) in response to receipt of a second digital output and a second analog output, sink current at a reference current having an amplitude determined by the second analog output; and maintaining the electrically conductive surface of the housing at a constant potential when any or all of the stimulation electrodes are sourcing or sinking current.
 14. The method of claim 13, further comprising causing each of the plurality of stimulation electrodes to source current through its associated stimulation electrode for a first period of time determined by the first digital output.
 15. The method of claim 13, further comprising causing each of the plurality of stimulation electrodes to sink current through its associated stimulation electrode for a second period of time determined by the second digital output
 16. The method of claim 13, further comprising causing each of the plurality of stimulation electrodes, through individual programmed instructions comprising a digital output and an analog output, to have a high impedance in response to receipt of a third digital output.
 17. The method of claim 13, further comprising causing each of the plurality of stimulation electrodes, through individual programmed instructions comprising a digital output and an analog output, to short in response to receipt of a fourth digital output. 